This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Although there is widespread evidence for the use of amorphous precursors in the synthesis of shaped biominerals, the mechanisms of biological synthesis and stabilization remain poorly understood. Research to date has focused largely on the role of additives and organic interfaces in providing transient stabilization. A recent study indicates, however, that amorphous calcium carbonate (ACC) particles formed in bulk appear to be stable below a diameter of 70 nm [1]. The lower surface energy of a meta-stable phase compared to itscrystalline polymorphs could conceivably lead to a threshold size below which the amorphous form is actually the most stable. In mineralizing cells, small vesicles are very likely involved in ACC synthesis, transport, and storage, adding an additional layer of complexity. The small size of the vesicles (30-300 nm), the lipid surface chemistry, and the high radius of curvature may each play an essential role in achieving stability by modulating surface interactions. Here we use an in vitro system, aqueous calcium salts encapsulated within 0.1 - 1 micron diameter unilamellar vesicles, to study this problem. Precipitation is initiated by addition of ammonium carbonate. Preliminary small and wide angle x-ray scattering (SAXS/WAXS) data indicate stabilization of ACC nanoparticles in the range from 10 to 300 nm for at least 13 hours. To extend our analysis, we use SAXS to study precipitation and aggregation of encapsulated ACC nanoparticles. In contrast to bulk precipitation, isolation of the precipitates inside vesicles with limited ion concentration offers a unique control over crystal growth.